1. Technical Field
The present disclosure relates to a flat display device, and more particularly, to a liquid crystal display (LCD) device with a touch function which facilitates improved display and touch sensing performance, and a method for manufacturing the same.
2. Discussion of the Related Art
As a substitute for an input device such as a mouse or a keyboard according to the related art, a touch screen is widely used, wherein the touch screen facilitates direct input of information to a screen through the use of finger or pen/stylus. Thus, an application of the touch screen is gradually increased owing to an advantage of easy operation.
When the touch screen is applied to an LCD device, the touch screen is provided inside a thin film transistor (TFT) array substrate (lower substrate) of a liquid crystal panel to obtain slimness. To decrease a thickness of device, the touch screen and the liquid crystal panel are formed as one body.
For example, an in-cell touch type LCD device has been developed, wherein a common electrode on the TFT array substrate functions as a touch electrode, and a sensing line (RX line and TX line) for connection of the common electrode (touch electrode) is additionally formed.
FIG. 1 illustrates an LCD device according to the related art.
With reference to FIG. 1, an active area 10 for displaying images is formed on a TFT array substrate, and the active area 10 is provided with a plurality of pixels formed in a matrix configuration.
Also, a non-display area is formed in the periphery of the active area 10. At one side of the non-display area, there is a GIP (gate-in-panel) type gate driver 20 among driving circuits. At the other side of the non-display area, there is a pad 30 for connecting the active area 10 with the driving circuits.
FIG. 2 illustrates problems of the LCD device according to the related art.
With reference to FIG. 2, the pixel is defined by crossing a gate line 40 and a data line 50. Adjacent to a crossing portion of the gate line 40 and the data line 50, a thin film transistor (TFT) is formed.
For a common electrode to function as a touch electrode, a sensing line is formed on the TFT array substrate, to thereby connect the common electrodes in the X-axis direction and Y-axis direction.
For example, the sensing line may comprise a driving line (TX line) 60 supplied with a touch driving signal, and a receiving line (RX line) 70 for detecting a capacitance according to the touch driving signal.
For a display period, a common voltage (Vcom) is supplied to the common electrode, to thereby display images. For a non-display period, the common electrode is driven as the touch electrode, to thereby sense the change of capacitance according to a touch.
In order to improve reliability of LTPS (low temperature polysilicon)-based TFT and leakage current characteristics of TFT off-state, a lightly doped drain (LDD) region is formed by lightly doping source and drain regions of TFT with a dopant. In this case, driving reliability and current-voltage (I-V) characteristics of the TFT are largely influenced by an LDD length.
According to the reliability of TFT and TFT off-state characteristics, the LCD device according to the related art is designed to have the same LDD length both in the TFT of the active area and the TFT of the GIP buffer 22. Thus, an interference occurs between display driving and touch driving due to deterioration of ON-State Resistance (Ron) characteristics of the TFT.
The LDD of the TFT in the GIP buffer 22 of the in-cell touch type liquid crystal panel is formed to improve the reliability and TFT off-state characteristics. However, the LDD occupies 70-80% of the entire resistance of TFT, which causes the increase of series resistance between source and drain. Thus, while the TFT reliability and TFT off-state characteristics are improved by the LDD, the touch characteristics are deteriorated due to the deteriorated Ron characteristics.
In the in-cell type touch panel, structures for the touch driving are provided inside the liquid crystal panel, whereby the interference occurs between displaying characteristics and touch driving characteristics. Improving the Ron characteristics of the TFT decreases the interference between display driving and touch driving.
The touch driving signal for the touch driving leaks into a parasitic capacitance (CDTX, CGD, CDRX, CGRX, CMu) inside the panel, thereby causing noise. Improving the Ron characteristics of the TFT removes the noise.
For improvement of the Ron characteristics of TFT, the LDD length is decreased. However, since the LDD length has effects on the reliability of TFT and the TFT off-state characteristics, it is difficult to decrease the LDD length.
To improve the touch characteristic, the LDD length has to be designed in consideration of the decrease of leakage current in the active area 10 and the uniformity of parasitic capacitance. However, the related art does not overcome these problems.